This invention relates to methods and apparatus for monitoring combustion dynamics within the combustor stage of a gas turbine engine.
Gas turbines are extensively used in power plants for a wide diversity of applications including, as examples, electric power generators in utility power plants, land based engines for gas fired electrical generator or pipeline compressors, and shipboard or airborne engines for, respectively, marine or aeronautical propulsion.
Gas turbines burn hydrocarbon fuel which may include natural gas or kerosene, which is used as an aviation (jet) fuel. As a result of the combustion process, such turbines emit an exhaust stream containing a number of combustion products, including various forms of nitrogen oxide, collectively referred to as "NO.sub.x ", which is considered a pollutant.
It is widely known that, for a gas turbine, NO.sub.x emissions increase significantly as the combustion temperature rises. It is also known that operating a turbine in a so-called "lean burn" condition, which involves use of a lean mixture of fuel and combustion air (i.e., a relatively low fuel-to-air ratio), reduces the combustion temperature to a level that significantly reduces NO.sub.x emissions.
Brown et al. Pat. No. 5,257,496, issued Nov. 2, 1993, entitled "Combustion Control for Producing Low NO.sub.x Emissions Through Use of Flame Spectroscopy" and related Brown patent application Ser. No. 08/226,528, filed Apr. 12, 1994, also entitled "Combustion Control for Producing Low NO.sub.x Emissions Through Use of Flame Spectroscopy", both of which are assigned to the instant assignee, disclose closed loop feedback control systems which achieve a lean burn by employing a silicon carbide (SIC) photodiode to sense combustion temperature through measurement of the intensity of ultraviolet radiation from a combustion flame and continuously adjusting the fuel/air ratio of the fuel mixture such that the ultraviolet radiation intensity remains below a predetermined level associated with a desired low level of NO.sub.x emissions. The SiC photodiode, which is located behind a sapphire or quartz window, responds to ultraviolet emissions of the flame and thereby responds to the intensity or temperature of the flame, since the photocurrent produced by the photodiode is proportional to the photon flux produced by the flame and impinging on the photodiode.
As a separate (but related) consideration, what is known as combustion dynamics is a critical parameter of a gas turbine, and is closely monitored. During the combustion process, the fuel and air mixture is ignited and burned in the combustor, producing extremely hot gas at high pressure. Dynamic pressure waves having an acoustic frequency range of from a few hundred hertz to a few thousand hertz occur during the process. If these dynamic pressure waves are not maintained at a sufficiently low level, mechanical damage can result. Further, gas turbine life decreases when its vibration is excessive, and the turbine may become too dangerous to operate because of those vibrations.
Moreover, it has been observed that lean burn conditions, which are desirable from the point of view of achieving low NO.sub.x emissions, exacerbate the problem of dynamic pressure waves being produced. If these dynamic pressure waves are not held to a sufficiently low level, they result in undesirable mechanical vibrations. Dynamic pressure wave minimization is usually accomplished by increasing flame temperature, thereby stabilizing the flame front. As a result, however, NO.sub.x emission cannot reach design specifications.
Accordingly, the ability to diagnose and control the dynamics of a turbine in real time is of critical importance, and monitoring combustion dynamics during normal operation once a turbine has been installed is essential.
A current and accepted practice is to detect and measure the dynamic pressure waves by employing a pressure transducer, typically one which includes a piezoelectric crystal and a tube having one end projecting into the combustion chamber so as to be exposed to pressure therein. The piezoelectric crystal is mounted at the other end of the tube. The tube thus serves to reduce the amount of pressure applied to the piezoelectric crystal to prolong the life of the pressure transducer. This sensor design reflects the difficult environment of a gas turbine insofar as sensors are concerned, where both high pressure and high temperatures are involved.
To monitor combustion dynamics in essentially real time, a frequency spectrum analysis of the pressure transducer output signal is accomplished by performing a Fast Fourier Transform operation. The acoustic waves at various frequencies are indicated as peaks in the frequency spectrum analysis.
While such pressure transducers provide relevant information, there are a number of significant disadvantages. The piezoelectric crystal is quite fragile, and frequently fails, causing difficulties and delays in testing of new turbine designs. Moreover, although desirable in principle, pressure transducers in practice typically are not left in place on installed turbines. Such would be desirable for accomplishing "lifetime" tracking of flame dynamics during operation of the turbine.